This year, we made substantial, but as yet unpublished, progress on the several aspects of DNA mismatch repair. (1) We investigated the ability of yeast Pms1, an essential mismatch repair protein, to bind to DNA, and to interact with its protein partner, Mlh1. This investigation involved collaborations to solve the crystal structure of the Pms1 N-terminal domain, and to map the surface of Pms1 that interacts with DNA and with Mlh1, using chemical modification and mass spectrometry. (2) We initiated studies to determine the efficiency with which mismatches made during leading and lagging strand DNA replication in yeast are repaired. (3) We continued to investigate the roles of MutS alpha and exonuclease 1 in DNA mismatch repair in mice.